Java.util.HashSet Example

Introduction

Java HashSet class is used to create a collection that uses a hash table for storage. It inherits the AbstractSet class and implements Set interface.

A hash table stores information by using a mechanism called hashing.

HashSet stores the elements by using a mechanism called hashing.

In hashing, the informational content of a key is used to determine a unique value, called its hash code.

HashSet contains unique elements only.

Difference Between Set And List in Java

Declaration of java.util.HashSet

public class HashSet
       extends AbstractSet
       implements Set, Cloneable, java.io.Serializable
{
...
}

Example of java.util.HashSet

import java.util.HashSet;
import java.util.Iterator;

public class main {

    public static void main (String [] args)
    {
        //declare a default HashSet object
        HashSet set=new HashSet<>();

        //add elements to this set
        set.add("Item 1");
        set.add("Item 2");
        set.add("Item 3");

        System.out.println("number of elements in this set : " + set.size());

        System.out.println(set);
        //output : [Item 3, Item 2, Item 1]

        //return an iterator over the elements in this set
        Iterator iterator =set.iterator();

        while (iterator.hasNext())
        {
            System.out.println(iterator.next());
        }
    }
}

Output from The Program

number of elements in this set : 3
[Item 3, Item 2, Item 1]
Item 3
Item 2
Item 1

Java HashSet class